Electrostatic spray apparatus and method

ABSTRACT

An electrostatic spray apparatus and method for spraying electrically conductive coating materials on a continuous basis from an electrically grounded bulk coating supply. Included is a spray gun from which electrostatically charged coating material is emitted toward an electrically grounded object to be coated; a high voltage source for electrostatically charging the coating material emitted from the gun; an electrically grounded bulk coating storage tank; and an intermediate tank assembly including (a) an inner tank for continuously supplying coating to the gun via a hose, which inner tank is automatically and periodically replenished with coating material from the grounded bulk supply tank without establishing an electrically conductive path between the electrically grounded bulk storage tank and the inner tank which contains coating material electrostatically charged via the column of conductive coating material in the gun hose, (b) an electrically grounded outer housing or container completely enclosing the inner tank, and (c) an insulative support electrically isolating the inner tank from the outer grounded container, which support is subjected to a gaseous stream to prevent build-up thereon of a conductive film of coating material which, if permitted to occur, would shortcircuit the grounded outer container to the electrostatically charged coating material stored in the inner tank and thereby destroy the electrical isolation between the inner tank and the grounded outer container.

United States Patent [1 1 Tamny 1 1 ELECTROSTATIC SPRAY APPARATUS ANDMETHOD [75] Inventor: Simon Z. Tamny, Lorain, Ohio [73] Assignee:Nordson Corporation, Amherst,

Ohio

[221 Filed: Apr. 30, 1974 [21] Appl. No.: 465,539

Primary Examiner-Ronald Feldbaum Attorney, Agent, or Firm-Wood, Herron &Evans [57] ABSTRACT An electrostatic spray apparatus and method forHIG/l VULTAGE 1 July 1,1975

spraying electrically conductive coating materials on a continuous basisfrom an electrically grounded bulk coating supply. Included is a spraygun from which electrostatically charged coating material is emittedtoward an electrically grounded object to be coated; a high voltagesource for electrostatically charging the coating material emitted fromthe gun; an electrically grounded bulk coating storage tank; and anintermediate tank assembly including (a) an inner tank for continuouslysupplying coating to the gun via a hose, which inner tank isautomatically and periodically replenished with coating material fromthe grounded bulk supply tank without establishing an electricallyconductive path between the electrically grounded bulk storage tank andthe inner tank which contains coating material electrostatically chargedvia the column of conductive coating material in the gun hose, (b) anelectrically grounded outer housing or container completely enclosingthe inner tank, and (c) an insulative support electrically isolating theinner tank from the outer grounded container, which support is subjectedto a gaseous stream to prevent build-up thereon of a conductive film ofcoating material which, if permitted to occur, would shortcircuit thegrounded outer container to the electrostatically charged coatingmaterial stored in the inner tank and thereby destroy the electricalisolation between the inner tank and the grounded outer container.

22 Claims, 1 Drawing Figure VALVE 1 ELECTROSTATIC SPRAY APPARATUS ANDMETHOD This invention relates to electrostatic spray coating and moreparticularly to an apparatus and method for electrostatically spraycoating electrically conductive coating materials on a continuous basiswhich permits all exposed elements of the spraying apparatus to beelectrically grounded, thereby avoiding shock and ignition hazardsoccasioned by inadvertent contact by operating personnel with, and/oraccidental grounding of, any exposed portion of the system.

Coating materials sprayed on objects to be coated can be categorized,from the standpoint of their electrical conductivity, as falling intoone of three categories, namely, low intermediate or moderate, and highconductivity coatings. Coating resistivities in the general range of 2l0 ohm-centimeters to ohmcentimeters are considered to be in theintermediate or moderate conductivity range, while coating materialshaving electrical resistivities below and above this range are viewed asfalling in the high and low conductivity categories, respectively. Whilespecific resistivity values have been used to define low, intermediateand high conductivity range, it is understood that these resistivityvalues are arbitrary and relative, and employed only for the purpose ofillustration. Accordingly, a coating material having a resistivity aboveor below the range of 2 l0""--l0 ohm-centimeters could conceivably beconsidered as an intermediate or moderate conductivity coating material,notwithstanding that it falls without, although near, the specificnumerical range noted. Similarly, a coating material with a resistivitybe tween 2x10 ohm-centimeters and I0 ohmcentimeters, near one of theother of these limits, may possibly be considered as either a high orlow conduc' tivity coating depending on the end of the intermediaterange to which it is closest. As used herein, the term electricallyconductive coating material" refers to coating materials of both theintermediate and high conductivity types.

Heretofore there has been no commercially feasible method or apparatusfor spraying, on a continuous basis (vis-a-vis-batching operations), anelectrostatically charged flow of conductive coating material in whichall elements of the spray coating system exposed to the environment areelectrically grounded, thereby permitting an operator to contact anyexposed portion of the entire spray equipment system without risk ofelectrical shock and/or accommodating accidental grounding of anyexposed portion of the system without risk of ignition hazards shouldthe equipment be located in an explosive environment as is the case whenpetrochemical solvent-based paints are being sprayed.

In one spray system proposed in German Pat. No. 2,0 I 9,466 designed forcoating articles with electrically conductive paint, which is notsusceptive of continuous operation, the paint supply tank, which isconnected to the gun via a hose, is located on an electricallynonconductive stand or platform spaced above the floor (which is atground potential) at distance sufficient to avoid arcing or dielectricbreakdown of the air between the tank and floor. This is necessary as aconsequence of the fact that the contents of the tank are at thecharging potential by reason of the conductive column of paint in thehose which interconnects the spray gun electrode and the tank contents.With such an arrangement, and assuming the tank is fabricated ofelectrically conductive material which is not uncommon, the exposedsurface of the tank is electrostatically charged at the operatingpotential. An operator inadvertently contacting the tank risks seriouselectrical shock. Additionally, should a grounded object accidentallycontact the tank, there is presented a significant ignition hazard wherethe conductive paint is of the petrochemical solvent-based type, sincethe environment is likely to be explosive.

Of course, the foregoing shock and ignition hazards can be minimized bylimiting access to the tank by, for example, placing the tank in an areawhich is fenced off. However, this is inconvenient, adds to the cost ofthe system, and does not permit the tank to be conveniently moved whenthe spray equipment is utilized at different locations within the plant.

in accordance with the proposal of German Pat. No. 2,128,455 alsodirected to a batch-type operation, the ignition and shock hazardsattendant operator contact and/or accidental grounding of theelectrostatically charged paint tank are considered to be overcome bycompletely encasing the electrostatically charged paint tank within aninsulative housing, such as a tank fabricated of nonconductive materialof much larger size within which the electrostatically charged painttank can be placed.

However, a distinct disadvantage of each of the foregoing proposals isthat they permit only batch type, and not continuous, spraying by reasonof the fact that the amount of spraying which can be done during anygiven interval is limited by the size of the tank. Once the contents ofthe tank have been consumed it is necessary to interrupt the sprayingoperation, de-energize the electrostatic generator, electrically groundthe tank to discharge any electrostatic charge which accumulated on itduring the spraying operation, open the tank and replenish the tanksupply from a bulk storage paint supply such as from a large drum or thelike. After all this has been done, and then only, can the tank beclosed and the electrostatic spray coating operation resumed. This kindof operation is known as batchtype spraying, vis-a-vis continuous"spraying.

Furthermore, a large nonconductive enclosure immediately surrounding ahigh voltage electrostatic source, as in the second of the foregoingproposals, tends to accumulate charge inbalance on its inner and outersurfaces leading to a secondary shock hazard upon contact by anoperator.

In accordance with a further proposal described in US. Pat. No.3,122,320, it is suggested that a system for coating with electricallyconductive paint on a continuous basis (non-batch) from a firstelectrostatically charged tank which feeds the gun can be provided byreplenishing the conductive paint in the first tank from a secondgrounded bulk supply tank which sprays the paint into the first tankthereby presumably avoiding electrical continuity between the bulksupply tank which is grounded and the tank connected to the gun which ischarged. While this proposal might conceivably permit conductive paintto be electrostatically sprayed on a continuous basis, it does notovercome the shock and ignition hazards which attend inadvertent contactof the exposed electrostatically charged portions of the system by anoperator and/or grounded object.

Accordingly, it has been objective of this invention to provide anapparatus and method for electrostatically spraying conductive paint ona continuous (non-batch) basis free of shock and ignition hazardsoccasioned by r advertent contact of exposed portions of the apparatusby an operator and/or other grounded article. This objective has beenaccomplished in accordance with certain of the principles of thisinvention by supplying, via a hose, electrically conductive paint to agun from a paint tank which is enclosed within and spaced from an outerelectrically grounded tank-enclosing container, the spaced relationbeing provided by an insulative support member physically contactingboth the inner tank and outer container and over which a gas flow isdirected to prevent deposition thereon of an electrically conductivepaint film which if permitted to accumulate would provide a conductivepath between the electrostatically charged inner tank and the outerelectrically grounded container; and by further providing anelectrically grounded source of bulk coating material and materialconveying means interconnecting the bulk source and the inner tank fortransferring material therebetween without establishing an electricallyconductive path between the bulk source and the inner tank.

ln accordance with a preferred embodiment of the invention the innertank is open at the top and spaced from the outer grounded container bymeans of an insulative support extending between the spaced superimposedbases thereof, with the uncharged conductive coating material from theelectrically grounded bulk supply tank being introduced in adiscontinuous fashion into the inner tank from a hose passing throughthe top of the outer container. As a consequence of supplying conductivecoating material to the inner tank from the electrically grounded bulksupply without establish ing an electrically conductive path between thebulk source and the electrostatically charged material stored in theinner tank, continuous (non-batch) spray coating is possible utilizingan electrically grounded bulk paint supply.

Additionally, and by reason of directing a gas stream over theinsulative support which spaces the inner tank from the electricallygrounded outer container, an electrically conductive film of coatingmaterial originating from, for example, vaporized paint within theopentopped inner tank, is not permitted to accumulate on the tanksupport structure, which support structure constitutes the only directphysical link between the electrostatically charged inner container andthe electrically grounded outer container, thereby preventing thecreation of a short-circuit path between the inner electrostaticallycharged tank and the grounded outer container. A further advantage,attributable to placement at the bottom of the open-topped inner tank ofthe insulative support which spaces the inner tank from the outergrounded container, is that the distance between (a) the open top of theinner tank whereat conductive coating vapors concentrate and (b) thesupport which constitutes the only potential short-circuit path betweenthe inner and outer tanks, is maximized to further reduce thepossibility of a conductive film accumulating on the support andshort-circuiting the inner and outer tanks.

In accordance with a further aspect of the invention, the gas flow, inaddition to being directed over the insulative support which spaces theinner and outer tanks,

is also directed vertically upwardly in the annular region between theinner and outer tanks with a velocity which exceeds the downwardlydirected terminal velocity of vaporized coating particles or dropletstending to move downwardly toward the insulative tank support under theforce of gravity. This further enhances the likelihood that anelectrically conductive film will not be deposited on the supportlinking the inner and outer tanks.

In a preferred form of the invention an area restrictor in the form of anonconductive circular ring is mounted in the annular flow regionbetween the inner and outer tanks proximate the upper edge of the innertank. Such an area restrictor, when positioned in the manner indicated,increases the local upward velocity of the gas stream in the region ofthe inner tank opening, whereat particles or droplets of conductivecoating from the mist located above the stored coating in the inner tankwould tend to enter the annular flow region between the tanks in thecourse of moving downwardly toward the tank support.

In accordance with a further feature of the invention, the gas stream isprovided from a pressurized source of low moisture content gas via asuitable hose or the like. This has the dual advantage of pressurizingthe contents of the inner tank to provide a pressurized flow of coatingto the spray gun and promoting evaporation of any coating which mightcondense on the inner tank support.

Assuming the gas utilized to sweep the tank support and providepressurized coating flow is also non-toxic and nonexplosive, very simplemeans can be utilized to exhaust the gas from the outer container.Specifically, such gas flow exhaust means can take the form ofa simplevent provided in the top of the outer container above the inner tankthrough which the gas is vented, which is introduced at the bottom ofthe container, exhausts to the atmosphere. Such a gas exhaustarrangement does not create environmental pollution hazards nor does itcreate ignition hazards.

These and other advantages and objectives of the invention will becomemore readily apparent from a detailed description of the drawing inwhich the FIGURE is a schematic diagram of a preferred embodiment of acoating material spray system incorporating the principles of thisinvention.

The preferred embodiment, as shown in the FIG- URE, includes a spraydevice 10, preferably in the form of a hand-held spray gun, for emittingelectrostatically charged coating material 11, e.g., electricallyconductive water-based paint, toward an object 12 to be coated which ismaintained at an electrical potential substantially different from thatof the charged coating material emitted from the spray gun. In practice,the coating material 11 as emitted from the gun 10 is in atomizeddroplet or particulate form as is well known in the electrostaticcoating field. such atomization or the like being accomplished inaccordance with conventional airless spray, air spray, or electrostaticatomization principles The emitted material 11 is typicallyelectrostatically charged to a potential in the range of 50 kv kv,although potentials outside this range are possible, while the article12 to be coated is typically maintained at zero or ground potential asindicated by ground connection 61.

While the advantages provided by the instant invention are perhapsgreatest when a spray device of the manually held variety is used, it isto be understood that the invention can also be used with stationaryspray de vices secured to a fixed stationary support, as well as movablespray devices used in automatic spray systems in which the spray deviceis mounted on a movable support which reciprocates vertically and/orhorizontally in response to control signals generated by a suitableposition controller.

The gun includes an elongated barrel [3 fabricated of electricallyinsulative material and a handle 14 fabricated of electricallyconductive material. The spray gun handle 14 is customarily electricallygrounded as shown by ground connection G2. The longitudinal length ofthe insulative barrel 13, which establishes the distance between theemission point or nozzle 15, whereat the electrostatically chargedmaterial 11 leaves the gun, and the electrically conductive handle 14,should be selected to provide an electrical standof i.e., avoid arcing,Corona discharge, undesirable electrical shunt current levels and thelike, between the handle and the nozzle. Located within the gun is aconduit 16 through which paint, supplied to the gun via a hose 17 from aremote source to be described, flows to the nozzle 15. Connected in theconduit 16 is a paint flow valve 18 having an actuating plunger 20 whichreciprocates under the action of a movable fingeroperated trigger 21pivoted to the gun at 22. Inward and outward movement of the trigger 21reciprocates the plunger 20 to open and close the flow valve 18respectively, regulating the flow of paint from the gun is a manner tobe described. Also operated by the trigger 21 is a normallyopen-circuited electrical switch 23 which, when the trigger 21 isdepressed, is placed in a closed-circuit condition to initiate theapplication of an electrostatic charging potential to the paint in amanner also to be described hereafter.

To provide the capability of supplying the gun 10, via the hose orconduit 17, with electrically conductive paint on a continuous basis, anintermittently refillable intermediate tank assembly 24 and a bulkcoating supply tank 25, both of which are grounded, as indicated byground connections G3 and G4, are provided. The supply of electricallyconductive paint to the gun 10 on a continuous basis from anelectrically grounded paint supply, as is possible with the apparatusand method of this invention, is to be distinguished from what is knownas batch-type operation. In such operation the tank directly supplyingpaint to the gun via an interconnecting hose supplies the gun with paintonly until the contents of the tank are depleted, at which time it isnecessary to stop the painting operation, electrically discharge thetank, open the tank and refill it with a new supply of paint, followingwhich the tank is closed and the spraying of electrostatically chargedpaint resumed. The significance of the fact that electrically conductivepaint is supplied via hose 17 to the gun 10 on a continuous basis fromtanks 24 and 25 which are electrically grounded will become apparenthereaf ter. It is sufficient at this point to note only that byutilizing electrically grounded tanks 24 and 25 electrically conductivepaint can be supplied continuously by hose 17 to the gun 10 without needfor locating the tanks 24 and 25 on specially constructed dielectricplatforms in areas which have been fenced off or otherwise madeinaccessible for the purpose of avoiding electrical shock and ignitionhazards which attend exposure of high voltage equipment to operatingpersonnel and/or inadvertent grounding.

A high voltage electrostatic generator 26 of conven tional design,having a housing 27 electrically grounded as shown schematically byground connection G5, provides at its output conductor 28 anelectrostatic potential sufficient in magnitude to charge the pain. i1emit ted from nozzle 15 to the desired level. In a preferred form thehigh voltage output 28 is electrically connected via an insulatedelement 30 to an electrically conductive electrode in the form of a ring31 located within the bore 32 of the paint hose 17 such that paintflowing through the hose is electrostatically charged at the voltageoutput from the generator 26 on line 28. Since the paint flowing throughthe hose 17 is electrically conductive, the coating material 11 emittedfrom the nozzle 15 will be electrostatically charged to essentially thepotential of the ring 31 via the electrically conductive paint column inthe hose 17 between the electrode ring 31 and the nozzle 15. A suitableelectrically insulative fitting 33 surrounds and seals the insulatedelement 30 and the hose 17 in the region of the charging electrode ring31. A suitable control 34 is operatively connected to the high voltagegenerator 26 for controlling the ON and OFF state of the generator inresponse to actuation of the trigger 21 via electrical control wires 35,as will become more apparent hereafter.

While electrostatic charging of the emitted electrically conductivecoating material 11 is accomplished in the preferred embodiment bycharging the electrically conductive paint as it moves through the hose17, other methods are possible. For example, the emitted coatingmaterial 11 can be electrostatically charged in the region of the nozzle15 by mounting an electrode (not shown) in the vicinity of the nozzle,which electrode is electrically connected to the high voltage generatoroutput line 28 via a suitable insulated cable (not shown). However, suchan arrangement has the disadvantage of requiring that a cable, insulatedfor high voltage, be connected to the gun, making the gun morecumbersome to handle and increasing electrical shock and ignitionhazards occasioned by the added electrical energy capacitively stored inthe cable. However, this disadvantage of a gun-mounted high voltageelectrode can be overcome by mounting the high voltage generator withinthe gun as disclosed and claimed in Senay U.S. Pat. No. 3,73] ,145,assigned to the asstgnee of this application. A still furtheralternative method of electrostatically charging the electricallyconductive material 1 1 emitted from the gun 10 is to connect the output28 from the high voltage generator 26 to the tank containing the paintwhich feeds hose 17 such that it makes electrical contact with thecoating material therein, as described, for example, in U.S. Pat. No.3,794,243, assigned to the assignee of this application.

The hose 17, which transports electrostatically charged paint within itsbore 32 from the intermediate tank assembly 24 to the gun 10, preferablyhas an interior layer or zone 36 which is impermeable and chemicallyinert with respect to the electrically conductive paint flowing throughthe bore 32, an intermediate insulative layer 37 which, in combinationwith the layer 36, has a dielectric strength sufiicient to preventdielectric breakdown when subjected to the charging potential by theelectrostatically charged conductive paint flowing in the bore 32.Preferably surrounding and in physical contact with the intermediatelayer 37 is an electrically conductive element 38 which is grounded asschematically shown by ground connection G6. The grounded element 38prevents the accumulation of electrical charge on the exterior of thehose l7 occasioned by minor current leakage radially through the hosefrom the electrostatically charged paint flowing therethrough.Accumulation of leakage charge on the exterior of the hose 17, whichmight otherwise occur were the electrically grounded conductive element38 not provided, presents an ignition and/or electrical shock hazardsshould it accidentally become grounded in an explosive environmentand/or contact an operatori The bulk coating supply tank 25 which asnoted previously is grounded at G4 can be fabricated of any suitablematerial and be shaped in any suitable form. Preferably the tank 25 hasa large storage capacity, for example, 100 gallons or more. Locatedwithin tank 25, in which is stored a large quantity of electricallyconductive paint P, is a paint circulating pump 43 having a suction end42 located in the bottom region of the tank. The pump 43 provides apressurized flow of paint in the tube 41 in the direction of arrow 45.interconnected in the paint hose 4] is a suitable flow control valve 46for regulating the flow of paint in hose 41 in response to a controlinput on lines 47 to an associated electrical control 48, such as asolenoid, derived in a manner to become apparent hereafter. The paint Pcan be replen ished as needed through opening 40.

The intermediate tank assembly 24 is supplied with coating material fromthe bulk supply 25 under the control of valve 46 when the coating supplyin tank assembly 24 reaches a predetermined depletion level.Intermediate tank assembly 24 includes an inner coating materialcontainer or tank 50, and an outer container, enclosure or tank 51 whichencloses the inner tank. The outer tank 51 is preferably constructed ofelectrically conductive material, and as previously noted iselectrically grounded as shown schematically at G3. The interior of theouter tank 51 is accessible from the outside for assembly andmaintenance purposes via an opening in the top thereof wich is normallyclosed with a selectively removable cover 52 which passes the painttransporting hose 4! from bulk supply 25. The inner tank 50, which holdsor stores a supply of electrically conductive paint P, is preferablyfabricated of electrically conductive material. Extending from thebottom 53 of the tank 50 and communicating with the tank interior is oneend 54 of an insulative helical paint conveying tube or hose 55, theother end 56 of which connects to the end of the paint hose 17 passingthrough a suitable located sealing grommet 57 positioned in an openingin the lower section of wall 58 of the outer tank 51. The helical tube55 transports electrostatically charged paint from the interior ofcontainer 50 to the hose 17 for transmission to the gun 10. Since theelectrically conductive paint within the bore 32 of the tube 17 iselectrically charged at the potential of the output 28 of high voltagegenerator 26, the paint P within the tank 50 is also at theelectrostatic charging potential by reason of the electricallyconductive paint column in hose 55 which interconnects the hose 17 withthe inte rior of the inner tank 50.

To facilitate replenishing the supply of paint in inner tank 50 (whichis at the electrostatic charging potential) from the bulk coating supplytank 25 (which is at grounded potential) via the hose 4] without placingthe contents P of bulk coating supply tank 25, and in turn the tank 25itself, at the electrostatic charging potential, the transfer of paintfrom the end 60 of the tube 41 to the inner tank 50 must be accomplishedwithout establishing an electrically conductive path between the paintissuing from hose end 60 and the electrostatically charged paint Plocated in the inner tank 50. To accomplish this, in accordance with apreferred embodiment of the invention, a nozzle 61 secured to the end 60of the hose 4! is provided to divide the flow of paint issuing from hose60 into an electrically discontinuous spray of particulate or dropletform, as shown schematically by the discrete particles 62. lf electricaldiscontinuity is to be obtained between the paint issuing from hose end60 and the paint P in the inner container 50 by use of a spray nozzle61, a coarse spray is preferable to a fine spray or mist.

Alternatively, electrical discontinuity between the material flowingthrough hose end 60 and the paint P' in the inner container 50 can beobtained without use of a spray nozzle 61 by discharging the paint fromthe hose end 60 in timed, spaced, successive discrete quantities orpulses. Such could be accomplished by operating the control 48, whichactuates the valve 46, in a pulsating mode in accordance with techniqueswellknown in the art.

To avoid arcing, Corona discharge and the like between the paint issuingfrom hose end 60 (or nozzle 61) which is at electrical ground potentialand the paint P in the inner container 50 which is at the electrostaticcharging potential, it is necessary to position the hose end 60 (ornozzle 6!) a distance from the maximum contemplated level of the paint Psufficient to provide a suitable electrical standoff. Similarly, toavoid arcing, etc. between the ground potential paint issuing from hoseend 60 (or nozzle 61) and the inner container 50 which is in contactwith the electrostatically charged paint P, the shortest distancebetween the vertical side wall 63 and/or the upper edge 64 of the innercontainer 50 and the hose end 60 (or nozzle 6]) must be sufficient toprovide an electrical standoff. it is also essential to space the upperedge 64 of the inner container 50 which is at high electrostaticpotential from the electrically grounded side wall 58 and cover 52 ofthe outer container 51 a sufficient distance to provide an electricalstandoff condition.

To further reduce the possibility of arcing, Corona discharge and thelike between the inner container edge 64 which is electrostaticallycharged and the ground potential outer container wall 58 and cover 52,the upper edge 64 should be smoothly contoured to avoid sharp edgeswhich increase the concentration of the electrostatic field thereat.

It is also desirable to space the electrostatically charged verticalside wall 63 and the grounded outer container wall 58 from each other adistance sufficient to avoid arcing, Corona discharge and the like.Similarly, the electrostatically charged inner container bottom 53 andthe grounded outer container bottom 65 should be spaced apart a distancesufficient to provide a suitable electrical standoff.

For example, at a typical electrostatic potential of kv a standoff ofapproximately 6 inches between the conductive surfaces of the inner tank50 and outer tank 51 may be expected to limit the average leakagecurrent from tank 50 to tank 5] to less than 25 microamps. Efficientelectrostatic coating requires that the power supply provide both a highpotential to establish the necessary electrostatic field between the gunand work, as well as a current flow sufficient to electrically chargepaint particles emitted from the spray gun. A further demand is imposedon the power supply by current leakage from the charged system to earthgrounds other than the work piece. The power supply therefore must, ifefficient coating is to be achieved, have sufficient capability to meeteach of the foregoing demands. There is, however, a practical limit onpower supply capability due to personnel hazards which are created asthe power supply capability is increased beyond safe limits. To minimizethis safety hazard, without deteriorating coating efficiency, currentleakage must be held to a minimum.

To support and locate the inner container 50 relative to the outercontainer 51 as desired, a support structure 66 in the form of avertically disposed column of insulative material is secured in fixedrigid relation to the outer container bottom 65. The lower end 67 of theinsulative support column 66 preferably takes the form of a threadedstub which extends downwardly through an opening 68 provided in acup-shaped central section 69 of the outer tank bottom 65. A nut 71threads on the projecting portion of the stub 67 to lock the column 66in its desired upright position. A grommet 72 disposed between theopening 68 and the threaded stub 67 seals the opening when the stub islocked in place with the threaded nut 71.

The upper end of the column 66 is provided with a tubular section 73which telescopes within an inverted cup 74, preferably fabricated ofconductive material, which snugly fits within an inverted recessedsection 75 formed in the bottom 53 of the inner tank 50. A conductivecompression spring 76 positioned within the tubular section 73 of thevertical support 66, for reasons to become apparent hereafter, appliesan upward force to the bottom 53 of the inner tank 50.

Positioned within a vertical axially disposed bore 77 provided in thesupport column 66 is a longitudinal insulative element or rod 78, theupper end of which contacts the uppermost section 80 of the inverted cup74. The bottom end 8] of the rod 78 contacts a lever 82 pivotallymounted at 83 which is upwardly biased by a compression spring 84.Mounted on the lever 82 is a switch 85.

When the supply of paint P in the inner container 50 reaches somepredetermined depletion or low level and the inner container 50 hasrisen upwardly under the action of spring 76 to a specified point, thelever 82 hiased by spring 84 pivots clockwise about spatially fixedpoint 83 causing switch 85 to complete a circuit through wires 47. This,in turn, causes the control 48 to open the valve 46, whereupon groundpotential paint from the grounded bulk coating supply tank 25 issupplied via the hose 41 to the interior of the inner tank 50 toreplenish electrostatically charged paint supply P. As describedpreviously, replenishment of tank 50 from bulk supply 25 is accomplishedin an electrically discontinuous manner via the nozzle 61 which convertswhat could be an electrically continuous paint stream issuing from hoseend 60 into an electrically discontinuous coarse spray 62.

By techniques commonly known, the actuation of switch 85 is providedwith a dead band such that the inner tank 50 drops to a predeterminedlower level as a consequence of being replenished with paint from thebulk supply 25 before switch is disengaged to effect the closing ofvalve 46.

The detector or sensor for determining when it is necessary to replenishthe supply of paint in the inner tank 50 may take a variety of formsother than the rod 78 movable with the inner tank 50 and switcharrangement 82, 83, 84 and 85. For example, level detectors or sensorsoperating on pneumatic, magnetic, optical and like principles well-knownto those skilled in the art could be employed. The particular detectingor sensing scheme shown in the FIGURE and described herein isillustrative only.

To space the bottom 65 of the outer container 51 from the floor 86 orthe like, the outer tank 51 is provided with a downwardly extendingenclosure 87 which provides an accommodation space between the outercontainer bottom 65 and the floor 86 for the projecting end 81 of themovable actuating rod 78 and the switch 82, 83, 84 and 85. Of course, ifthe sensor used to detect the level of paint in the inner tank 50 werecontained within outer container 51, or located other than below thebottom 65 of outer tank 51, outer tank 51 could be placed directly onfloor 86, dispensing with the need for enclosure 87.

To provide a controlled bleed to ground potential of electrical chargeaccumulated on inner tank 50, and in the electrically charged paint Pwhen the high voltage generator 26 is de-energized, a bleed resistor 88is provided. Preferred resistance values for said bleed resistor wouldbe in the range of lOg-30g ohms, although a somewhat wider range wouldbe acceptable. Resistor 88 is connected between the grounded outercontainer 51 and an electrically conductive washer 90 disposed betweenthe bottom of the compression spring and the bottom of the tubularextension 73 of the support column 66. The bleed resistor 88 preferablyis embedded within and completely surrounded by the insulative supportcolumn 66, although this is not necessary. Alternatively, the bleedresistor 88 could be directly connected between the grounded outercontainer 51 and the inner container 50 at any point along the bottom 53or side walls 63 thereof. Of course, such an alternative resistorconnection must avoid surface short-circuiting and should providesuitable accommodation for relative movement between the inner and outercontainers 50 and 51 as occurs in the preferred embodiment wherein theinner container moves up and down depending upon the quantity of paint Pstored therein at any given time.

Since any electrically conductive paint stored in the inner container 50has a certain degree of volatility, a paint mist M will exist in thespace above the stored paint P due to volatization or evaporation of thestored paint P in tank 50. This mist M will produce an electricallyconductive paint film if allowed to deposit on the exterior surface 91of column 66 or the interface 92 between column 66 and rod 78 whichcollectively space the inner and outer tanks, which surfaces areadjacent to the space S which communicates with the mist of conductivepaint above the level of the paint P via annular path A between verticaltank walls 63 and 58. This electrically conductive paint film cancomplete an electrically conductive path between the outer tank 51 whichis grounded and the electrically charged paint P stored in the innertank. Such an electrically conductive path between the electricallycharged paint P stored in the inner container 50 and the electricallygrounded outer tank 51, if permitted to exist. would prevent electricalisolation of the electrostatically charged paint P from the groundedouter tank 51, resulting in an excessive current drain from theelectrostatically charged portion of the system.

To avoid the above-described electrical path between theelectrustatically charged paint P in tank 50 and the outer tank 51established by deposition of an electrically conductive film of paint onthe surfaces 91 and 92, a flow of gas in an upward direction isindicated by arrow 96 is established in the space S between tank bottoms53 and 65, and in the annular flow path A established by the verticalside wall 63 of inner tank 50 and the side wall 58 of outer tank 51.This upwardly directed gas flow 96 sweeps over tank support surface 91and through the annular path A, preventing the accumulation of anelectrically conductive paint film of the type described which, ifpermitted to occur, would electrically shortcircuit the electricallygrounded outer tank 51 to the electrically charged paint P stored in theinner tank 50.

To provide gas flow 96, the supply 44 of gas is connected via a tube 97to the space S between the inner and outer tank bottoms 53 and 65.Preferably the gas supply tube 97 is connected to the interior of theouter tank 51 at a point adjacent the bottom ofthe outer tank such thatthe entire exposed surface 91 of the tank support element 66 issubjected to the air flow 96. This maximizes the likelihood that anelectrically conductive film of paint originating with the paint mist Mexisting in the inner container 50 above the level of the charged paintP will not deposit an electrically conductive film on surfaces 91 and 92and thereby not establish a short-circuit path between the outer tank 51and the electrically charged stored paint P in the inner tank.

The gas flow 96 is exhausted from the interior of the outer tank 51 atthe top thereof via an exhaust path defined by an oversized bore 98formed in the cover 52 loosely surrounding the downwardly extendingpaint hose 41 and lateral vent holes 100 formed in a fitting 101 mountedto the upper surface of the cover. A bore 102 in the fitting 101 snuglyembraces the exterior of the paint hose 41.

The upward velocity of the gas stream 96 in the region of the edge 64 ofthe inner tank. 50 is preferably selected to exceed the downwardterminal velocity of paint particles from mist M entering the annularpath A between the confronting walls 58 and 63 of the outer and innertanks 51 and 50. This, in addition to the gas flow oversurface 91,insures that a paint particle entering the annular cavity A from mist Mat a point proximate the edge 64 will not reach the bottom region orspace S between the tank bottoms 65 and S3 to deposit on the surfaces 91and 92. To increase the local velocity of the gas flow 96 in the annularflow path A proximate the edge 64, a nonconductive flow area restrictor103 in the form of a circular ring having upper and lower beveled edgesis secured to the wall 58 of the outer tank 51 opposite the upper edge64 of the inner tank 50. The restrictor ring 103 produces a localizedreduction in the area of annular flow path A proximate inner tank edge64, thereby increasing the local velocity of the upwardly moving gasstream in the region of the edge 64.

Preferably the gas introduced into the space S and annular path A toestablish the gas flow 96 is relatively moisture-free, This promotesevaporation of any paint film which could conceivably be deposited onthe surfaces 91 and 92.

A sensor 104, in the form of a pressure responsive switch having amovable diaphragm 105 which communicates with the space S via a hose 106is provided. A normally open electrical switch 107 closes to connectwires 108 when the pressure in the space 8 reaches a level correlated tothe desired gas flow 96. Electrical bridging of wires 108 by thepressure-responsive switch 104, in series combination with electricalbridging of wires 35 by the trigger-actuated switch 23 occurring whenthe operator desires to paint the article 12, actuates the control 34 toenergize the high voltage generator 26 and in turn electrostaticallycharge the paint passing through the hose 27 via the ring electrode 31.Thus, in the preferred embodiment electrostatic charging of theelectrically conductive paint in the hose 17 which, as indicated, alsoelectrostatically charges the stored paint P via the paint column inhoses l7 and 55, can only occur when both the finger-actuated trigger 21has been depressed and the gas supply 44 is operative to provide the gasflow 96.

In the event that cover 52 of outer tank 51 is opened, providing accessto the electrostatically charged portion of the system, the pressure inthe space S is immediately reduced, opening the pressure-responsiveswitch 104 and deenergizing the high voltage generator 26. Theaccumulated electrostatic charge is then quickly bled off to groundthrough bleed resistor 88.

The gas source 44, in addition to establishing a gas flow 96 in thecontainer 51 via hose 97, additionally functions to pressurize the paintP stored within inner tank such that paint flows under pressure to thegun via hoses and 17. Of course, as an alternative to pressurizing theflow, or as a supplement to it, a fluid pump (not shown) mounted to thebottom 53 of the inner tank 50 could be interconnected in the hose 55adjacent the hose end 54 for pumping the charged paint from the innertank 50 to the gun via hoses 55 and 17.

Significantly, the tank support structure 66 which, if coated with afilm of conductive paint, would establish a short-circuit between theelectrically grounded outer tank 51 and the electrically charged paintP. is located between the bottoms 53 and of the inner and outer tanks 50and 51. This maximizes the distance between (a) the potentialshort-circuit conductive paint film on surfaces 91 and 92 and (b) themist M of conductive paint existing in the interior of the inner tank 50above the level of the electrostatically charged paint P, which mistconstitutes the source of the conductive paint film on surfaces 91 and92. Maximization of this distance between the location of the mist M ofelectrically conductive paint and the potential short-circuit pathcreatable by the paint film on surfaces 91 and 92 minimizes thelikelihood that the mist M will deposit on the surfaces 91 and 92 a filmof conductive paint, in turn minimizing the likelihood that ashort-circuit will occur between electrically grounded tank 51 and theelectrostatically charged paint P in the inner tank 50.

The helical shape of paint hose 55 between the bottom 53 of thevertically movable inner tank 50 and the stationary paint hose 17enables the inner tank 50 to freely shift in a vertical direction as thelevel of the paint P stored in the inner container 50 varies during usebetween replenishments of the inner tank via the valve-controlled bulkcoating supply tank 25.

While the invention has been described in connection with a preferredform of telescoping support 66, 74 for spacing and electricallyisolating the inner and outer tanks 50 and 51, while simultaneouslypermitting relative movement therebetween to accommodate the paint levelsensing function, other insulative support arrangements can be utilizedto electrically isolate the inner and outer tanks. Additionally, whilethe tank isolating support in the preferred embodiment is locatedbetween the bottoms 53 and 65 of the inner and outer tanks 50 and 51,the support may likewise be positioned elsewhere.

The preferred embodiment of the invention has utilized means in the formof an air sweep for volatilizing conductive liquid material, e.g.,condensed moisture and/or coating material, which may be deposited onthe exposed surfaces of the insulative support which spaces the innerand outer tanks. Other volatilizing conductive devices may, of course,also be used. For example, the auxiliary volatilizing means could takethe form of a heater embedded in the support. The heater would elevatethe temperature of the exposed surfaces of the support sufficiently tovolatilize liquid material deposited thereon. This in turn would preventbuild-up of an electrically conductive film of material on the exposedsurfaces of the support which, if permitted to occur, could electricallyconnect the inner and outer tanks.

Having described the invention, it is claimed:

1. An electrostatic coating system operable on a continuous basis forcoating articles with electrically conductive material supplied from anelectrically grounded bulk coating source comprising:

a material spray device for emitting electrostatically charged coatingmaterial for application to an article maintained at an electrostaticpotential different from that of said emitted coating material,

an electrically grounded source of uncharged bulk coating material,

an inner container for storing coating material within the interiorthereof, said inner container having an outer wall,

coating material transmission means for transferring uncharged coatingmaterial from said bulk source to said inner container withoutestablishing an electrically conductive path between said bulk sourceand said inner container,

an electrically grounded outer container enclosing said inner containerand spaced therefrom to provide a gas flow path between the inner wallthereof and said outer wall of said inner container, said gas flow pathcommunicating with said interior of said inner container,

a conduit interconnecting said inner container and said spray device forconveying coating material from said inner container to said spraydevice, said conduit being constructed of insulative material sufficientto withstand dielectric breakdown when conveying electrostaticallycharged coating material.

a high voltage source having an output connected for electrostaticallycharging said coating material emitted by said spray device,

an electrically nonconductive support structure physically contactingboth said inner and outer containers for supporting said inner containerin electrically isolated relation relative to said outer container, and

a source of gas connected to said gas flow path for providing a gas flowin said flow path to prevent deposition of a film of coating material onsaid support structure sufficient to conduct a s wstantial level ofelectrical current from said inner container to said outer container.

2. The system of claim 1 further including a detector responsive to thequantity of coating material in said inner container for actuating saidtransmission means to transfer uncharged coating material from said bulksource to said inner container when the quantity of coating material insaid inner container is less than a specified amount.

3. The system of claim 2 wherein said support in cludes resilient meanswhich permit movement of said inner container relative to said outercontainer to a degree correlated to the quantity of coating material insaid inner container, and wherein said detector is responsive to aspecified degree of movement of said inner container to effect saidtransfer of uncharged coating material from said bulk source to saidinner container when the quantity of coating material in said innercontainer is less than said specified amount.

4. The system of claim 3 wherein said detector includes an elementmovable with said inner container and an element sensor immovablerelative to said outer container for sensing movement of said innercontainer relative to said outer container for effecting saidtransmission of uncharged material from said bulk source to said innercontainer when the quantity of coating material in said inner containeris less than said specified amount.

5. The system of claim 4 wherein said element is housed within saidsupport and thereby unexposed to said interior of said inner containerto prevent deposition of coating material thereon.

6. The system of claim 1 further including an electrically conductiveouter member surrounding and physically contacting at least a portion ofsaid insulative conduit, said outer member being electrically groundedto avoid accumulation of electrical charge on the exterior of saidinsulative conduit.

7. The system of claim 1 wherein said uncharged coating materialtransmission means includes means to emit uncharged coating material tothe said interior of said inner container in a physically discontinuousflow.

8. The system of claim 7 wherein said uncharged coating material emitteris constructed to emit uncharged coating material in spray form to saidinterior of said inner container.

9. The system of claim 7 wherein said uncharged coating material emitteris constructed to emit uncharged coating material in sequential discretebursts which are spaced in time to avoid electrical continuity betweensuccessive discrete bursts.

10. The system of claim 1 further including an electrical resistorelectrically connected between said inner container and said groundedouter container to bleed electrical charge at a controlled rate fromsaid inner container to said outer container.

11. The apparatus of claim 1 wherein said inner container has an openingin the upper region thereof communicating with said gas flow paththrough which coating material passes downwardly into said innercontainer interior from said coating material transmission means locatedthereabove, and wherein said support structure physically contacts saidinner container in the lower region thereof, and wherein said gas sourceconnects to said gas flow path proximate said lower region of said innercontainer.

12. The apparatus of claim 11 further including an area restrictionlocated in said gas flow path to increase the velocity of said gassufficiently to prevent downward movement of coating material from saidinner container opening to said support.

13. The apparatus of claim 11 wherein said opening in said upper regionof said inner container is defined by a smoothly contoured margin ofsaid inner container wall free of sharp edges to minimize electricaldischarge when said inner container is subjected to said electrostaticcharging potential.

14. The apparatus of claim 1 wherein said inner and outer containers arespaced apart a distance sufficient to avoid electrical corona dischargetherebetween when said inner container is subjected to saidelectrostatic charging potential.

15. The system of claim 1 wherein said gas source provides substantiallymoisture-free gas to said gas flow path to enhance evaporation ofcoating material located in said gas flow path.

16. The system of claim 1 wherein said outer container further includesgas exhaust means for exhausting gas from the interior of said outercontainer following passage through said gas flow path.

17. The system of claim 1 further including a sensor communicating withsaid gas flow path for preventing energization of said high voltagesource in the absence of said gas flow in said gas flow path.

18. The apparatus of claim 1 wherein said gas source provides apressurized gas to said gas flow path to pressurize the interior of saidouter container and promote pressure flow of coating material to saidspray device via said conduit.

19. An electrostatic coating system operable on a continuous basis forcoating articles with electrically conductive liquid material suppliedfrom an electrically grounded bulk coating source comprising:

a material spray device for emitting electrostatically charged coatingmaterial for application to an article maintained at an electrostaticpotential different from that of said emitted coating material,

an electrically grounded source of uncharged bulk coating material,

an inner container for storing coating material within the interiorthereof, said inner container having an opening in the upper regionthereof communicat ing with the interior of the container, an outerwall, and a bottom,

coating material transmission means for transferring uncharged coatingmaterial from said bulk source to said inner container via said openingwithout establishing an electrically conductive path between said bulksource and said inner container,

an electrically grounded outer container enclosing said inner containerand spaced therefrom to provide a gaseous region between the inner wallthereof and said outer wall of said inner container, said gaseous regioncommunicating with said interior of said inner container via saidopening,

a conduit interconnecting said inner container and said spray device forconveying coating material from said inner container to said spraydevice, said conduit being constructed of insulative material sufficientto withstand dielectric breakdown when conveying electrostaticallycharged coating material,

a high voltage source having an output connected for electrostaticallycharging coating material emitted by said spray device, and

an electrically nonconductive support structure exposed to said gaseousregion and physically contacting both said inner and outer containersproxi mate said bottom of said inner container for supporting said innercontainer in electrically isolated relation relative to said outercontainer at a point remote from said opening in said inner container tomaximize the distance within said gaseous region between said supportand said inner container opening.

20. The apparatus of claim 19 further including means associated withsaid support to volatilize liquid material deposited on said support tothereby prevent accumulation of a conductive surface film on saidsupport structure sufficient to conduct a hazardous level of electricalcurrent from said inner container to said outer container.

21. The apparatus of claim l9 further including means for establishingin said gaseous region a barrier which does not physically contact saidinner container to impede migration of liquid material, and wherein saidbarrier means is located between said support and said inner containeropening.

22. An electrostatic coating system operable on a continuous basis forcoating articles with electrically conductive material supplied from anelectrically grounded bulk coating source comprising:

a material spray device for emitting electrostatically charged coatingmaterial for application to an article maintained at an electrostaticpotential different from that of said emitted coating material,

an inner container for storing coating material within the interiorthereof, said inner container having an outer wall,

coating material transmission means for transferring uncharged coatingmaterial from said bulk source to said inner container withoutestablishing an electrically conductive path between said bulk sourceand said inner container,

an electrically grounded outer container enclosing said inner containerand spaced therefrom, to provide a gaseous region between the inner wallthereof and said outer wall of said inner container, said gaseous regioncommunicating with said interior of said inner container,

a conduit interconnecting said inner container and said spray device forconveying coating material from said inner container to said spraydevice, said conduit being constructed of insulative material sufficientto withstand dielectric breakdown when conveying electrostaticallycharged coating material,

a high voltage source having an output connected for electrostaticallycharging coating material emitted by said spray device,

an electrically nonconductive support structure physically contactingboth said inner and outer containers for supporting said inner containerin electrically isolated relation relative to said outer container, saidsupport structure exposed to said gasesaid support structure sufficientto conduct a hazous region, and

means associated with said support to volatilize liquid materialdeposited on said support to thereby prevent accumulation of aconductive surface film on ardous level of electrical current from saidinner container to said outer container.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 13,892,357

DATED July 1, 1975 tNvENTORtS) 1 Simon Z. Tamny It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 5, line 30, "is" should be --in.

Column 7, line 44, insert the word "through" after "cover 52".

Signed and Scaled this C. MARSHALL DANN RUTH C. MASON CommissionerufParenrs and Trademarks Arresting Officer

1. An electrostatic coating system operable on a continuous basis forcoating articles with electrically conductive material supplied from anelectrically grounded bulk coating source comprising: a material spraydevice for emitting electrostatically charged coating material forapplication to an article maintained at an electrostatic potentialdifferent from that of said emitted coating material, an electricallygrounded source of uncharged bulk coating material, an inner containerfor storing coating material within the interior thereof, said innercontainer having an outer wall, coating material transmission means fortransferring uncharged coating material from said bulk source to saidinner container without establishing an electrically conductive pathbetween said bulk source and said inner container, an electricallygrounded outer container enclosing said inner container and spacedtherefrom to provide a gas flow path between the inner wall thereof andsaid outer wall of said inner container, said gas flow pathcommunicating with said interior of said inner container, a conduitinterconnecting said inner container and said spray device for conveyingcoating material from said inner container to said spray device, saidconduit being constructed of insulative material sufficient to withstanddielectric breakdown when conveying electrostatically charged coatingmaterial, a high voltage source having an output connected forelectrostatically charging said coating material emitted by said spraydevice, an electrically nonconductive support structure physicallycontacting both said inner and outer containers for supporting saidinner container in electrically isolated relation relative to said outercontainer, and a source of gas connected to said gas flow path forproviding a gas flow in said flow path to prevent deposition of a filmof coating material on said support structure sufficient to conduct asubstantial level of electrical current from said inner container tosaid outer container.
 2. The system of claim 1 further including adetectOr responsive to the quantity of coating material in said innercontainer for actuating said transmission means to transfer unchargedcoating material from said bulk source to said inner container when thequantity of coating material in said inner container is less than aspecified amount.
 3. The system of claim 2 wherein said support includesresilient means which permit movement of said inner container relativeto said outer container to a degree correlated to the quantity ofcoating material in said inner container, and wherein said detector isresponsive to a specified degree of movement of said inner container toeffect said transfer of uncharged coating material from said bulk sourceto said inner container when the quantity of coating material in saidinner container is less than said specified amount.
 4. The system ofclaim 3 wherein said detector includes an element movable with saidinner container and an element sensor immovable relative to said outercontainer for sensing movement of said inner container relative to saidouter container for effecting said transmission of uncharged materialfrom said bulk source to said inner container when the quantity ofcoating material in said inner container is less than said specifiedamount.
 5. The system of claim 4 wherein said element is housed withinsaid support and thereby unexposed to said interior of said innercontainer to prevent deposition of coating material thereon.
 6. Thesystem of claim 1 further including an electrically conductive outermember surrounding and physically contacting at least a portion of saidinsulative conduit, said outer member being electrically grounded toavoid accumulation of electrical charge on the exterior of saidinsulative conduit.
 7. The system of claim 1 wherein said unchargedcoating material transmission means includes means to emit unchargedcoating material to the said interior of said inner container in aphysically discontinuous flow.
 8. The system of claim 7 wherein saiduncharged coating material emitter is constructed to emit unchargedcoating material in spray form to said interior of said inner container.9. The system of claim 7 wherein said uncharged coating material emitteris constructed to emit uncharged coating material in sequential discretebursts which are spaced in time to avoid electrical continuity betweensuccessive discrete bursts.
 10. The system of claim 1 further includingan electrical resistor electrically connected between said innercontainer and said grounded outer container to bleed electrical chargeat a controlled rate from said inner container to said outer container.11. The apparatus of claim 1 wherein said inner container has an openingin the upper region thereof communicating with said gas flow paththrough which coating material passes downwardly into said innercontainer interior from said coating material transmission means locatedthereabove, and wherein said support structure physically contacts saidinner container in the lower region thereof, and wherein said gas sourceconnects to said gas flow path proximate said lower region of said innercontainer.
 12. The apparatus of claim 11 further including an arearestriction located in said gas flow path to increase the velocity ofsaid gas sufficiently to prevent downward movement of coating materialfrom said inner container opening to said support.
 13. The apparatus ofclaim 11 wherein said opening in said upper region of said innercontainer is defined by a smoothly contoured margin of said innercontainer wall free of sharp edges to minimize electrical discharge whensaid inner container is subjected to said electrostatic chargingpotential.
 14. The apparatus of claim 1 wherein said inner and outercontainers are spaced apart a distance sufficient to avoid electricalcorona discharge therebetween when said inner container is subjected tosaid electrostatic charging potential.
 15. The system of claim 1 whereinsaid gas source provides subsTantially moisture-free gas to said gasflow path to enhance evaporation of coating material located in said gasflow path.
 16. The system of claim 1 wherein said outer containerfurther includes gas exhaust means for exhausting gas from the interiorof said outer container following passage through said gas flow path.17. The system of claim 1 further including a sensor communicating withsaid gas flow path for preventing energization of said high voltagesource in the absence of said gas flow in said gas flow path.
 18. Theapparatus of claim 1 wherein said gas source provides a pressurized gasto said gas flow path to pressurize the interior of said outer containerand promote pressure flow of coating material to said spray device viasaid conduit.
 19. An electrostatic coating system operable on acontinuous basis for coating articles with electrically conductiveliquid material supplied from an electrically grounded bulk coatingsource comprising: a material spray device for emittingelectrostatically charged coating material for application to an articlemaintained at an electrostatic potential different from that of saidemitted coating material, an electrically grounded source of unchargedbulk coating material, an inner container for storing coating materialwithin the interior thereof, said inner container having an opening inthe upper region thereof communicating with the interior of thecontainer, an outer wall, and a bottom, coating material transmissionmeans for transferring uncharged coating material from said bulk sourceto said inner container via said opening without establishing anelectrically conductive path between said bulk source and said innercontainer, an electrically grounded outer container enclosing said innercontainer and spaced therefrom to provide a gaseous region between theinner wall thereof and said outer wall of said inner container, saidgaseous region communicating with said interior of said inner containervia said opening, a conduit interconnecting said inner container andsaid spray device for conveying coating material from said innercontainer to said spray device, said conduit being constructed ofinsulative material sufficient to withstand dielectric breakdown whenconveying electrostatically charged coating material, a high voltagesource having an output connected for electrostatically charging coatingmaterial emitted by said spray device, and an electrically nonconductivesupport structure exposed to said gaseous region and physicallycontacting both said inner and outer containers proximate said bottom ofsaid inner container for supporting said inner container in electricallyisolated relation relative to said outer container at a point remotefrom said opening in said inner container to maximize the distancewithin said gaseous region between said support and said inner containeropening.
 20. The apparatus of claim 19 further including meansassociated with said support to volatilize liquid material deposited onsaid support to thereby prevent accumulation of a conductive surfacefilm on said support structure sufficient to conduct a hazardous levelof electrical current from said inner container to said outer container.21. The apparatus of claim 19 further including means for establishingin said gaseous region a barrier which does not physically contact saidinner container to impede migration of liquid material, and wherein saidbarrier means is located between said support and said inner containeropening.
 22. An electrostatic coating system operable on a continuousbasis for coating articles with electrically conductive materialsupplied from an electrically grounded bulk coating source comprising: amaterial spray device for emitting electrostatically charged coatingmaterial for application to an article maintained at an electrostaticpotential different from that of said emitted coating material, an innercontainer for storing coating Material within the interior thereof, saidinner container having an outer wall, coating material transmissionmeans for transferring uncharged coating material from said bulk sourceto said inner container without establishing an electrically conductivepath between said bulk source and said inner container, an electricallygrounded outer container enclosing said inner container and spacedtherefrom, to provide a gaseous region between the inner wall thereofand said outer wall of said inner container, said gaseous regioncommunicating with said interior of said inner container, a conduitinterconnecting said inner container and said spray device for conveyingcoating material from said inner container to said spray device, saidconduit being constructed of insulative material sufficient to withstanddielectric breakdown when conveying electrostatically charged coatingmaterial, a high voltage source having an output connected forelectrostatically charging coating material emitted by said spraydevice, an electrically nonconductive support structure physicallycontacting both said inner and outer containers for supporting saidinner container in electrically isolated relation relative to said outercontainer, said support structure exposed to said gaseous region, andmeans associated with said support to volatilize liquid materialdeposited on said support to thereby prevent accumulation of aconductive surface film on said support structure sufficient to conducta hazardous level of electrical current from said inner container tosaid outer container.